Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a molecular system. In a Raman spectroscopic experiment, a monochromatic beam of light, typically in the ultraviolet, visible, or infrared regions of the electromagnetic spectrum, passes through a sample of molecules and a spectrum of scattered light is emitted. The term “light” refers to electromagnetic radiation having wavelengths within the visible and non-visible portions of the electromagnetic spectrum, such as the ultraviolet and infrared portions of the spectrum. The spectrum of light emitted from the molecule is called a “Raman spectrum” and the scattered light is also called “Raman scattered light.” A Raman spectrum can reveal electronic, vibrational, and rotational energies levels of a molecule. Different molecules produce different Raman spectrums that can be used like a fingerprint to identify molecules and even determine the structure of molecules. For example, Raman gas analyzers have many practical applications such as providing real-time monitoring of molecular changes in gas mixtures.
The Raman scattered light generated by a compound (or ion) adsorbed on or within a few nanometers of a structured metal surface can be 103-106 times greater than the Raman scattered light generated by the same compound in solution. This surface-enhanced Raman scattering (“SERS”) is strongest on silver (“Ag”), gold (“Au”), and copper (“Cu”) surfaces. SERS arises from two mechanisms. The first mechanism is an enhanced electromagnetic field produced at the surface of a metal. When the wavelength of incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface, excited electronic state called a “surface plasmon.” Molecules adsorbed or in close proximity to the surface experience a relatively strong electromagnetic field. Molecular vibrational modes directed normal to the surface are most strongly enhanced. The intensity of the surface plasmon resonance is dependent on many factors including the wavelength of the incident light and the morphology of the metal surface. The second mode of enhancement occurs from the formation of a charge-transfer complex between the surface and a molecule absorbed to the surface. The electronic transitions of many charge transfer complexes are typically in the visible range of the electromagnetic spectrum.
In recent years, SERS has emerged as a routine and powerful tool for investigating molecular structures and characterizing interfacial and thin-film systems, and even enables single-molecule detection. In spite of its recent popularity, SERS does have limitations, including strict requirements that must be met in order to achieve optimal enhancement, which is usually extremely non-uniform over a SERS-active substrate. One critical aspect of SERS involves producing an ideal reproducible surface morphology for maximum field enhancement that is uniform over the active substrate. However, achieving an ideal reproducible surface morphology with homogeneous high performance (high and uniform enhancement factor) has been quite daunting and elusive. In addition, typical optical systems for performing Raman spectroscopy is very large and consists of an optical microscope that focuses light from a source onto an analyte and the Raman spectrum emitted from the analyte is gathered through the same optical system. Collecting an emission spectrum in this manner is inefficient and these optical systems are often bulky. Thus, engineers, physicists, and chemists continue to seek improvements in substrate surface morphology and improvements in systems for performing surface enhanced Raman spectroscopy.